Fixes piglit test glsl-override-builtin. The linker incorrectly found
the prototype for the float signature, rather than adding a new
prototype with the int return type. This caused ir_calls with type int
to have their callees set to the float signature, triggering an assert.
In most cases, we needed to be reparenting the cloned IR to a
different context (for example, to the linked shader instead of the
unlinked shader), or optimization before the reparent would cause
memory usage of the original object to grow and grow.
Otherwise, ir_function_inlining will see the body of the function from
the unlinked version of the shader, which won't have had the lowering
passes done on it or linking's variable remapping.
Instead of using ir_call::callee, search for the signature in the
linked shader. This will allow resolving calls from functions
imported from other shaders. The ir_call::callee pointer in the
imported function will still reference a signature in the original shader.
The list of shaders to search needs to be provided as an explicit
parameter to support coming changes. At that point there is no reason
for it to be in the class. Also, fix some of the 'const' decorators.
